BiFeO₃ is a highly promising lead-free ferroelectric material, surpassing most conventional ferroelectric materials in terms of the polarization and Curie temperature, offering a pathway for potential applications at elevated temperatures. Nevertheless, challenges arise due to strong clamping effect of substrate, large coercive fields, and high leakage currents, causing BiFeO₃ films difficult to be polarized. The implementation of self-polarization presents a viable solution. Herein, we prepared BiFeO₃, up-graded films (which transition from BiFeO₃ to Bi_0.80Ca_0.20FeO_2.90 from the substrate to the film surface), and down-graded films (which transition from Bi_0.80Ca_0.20FeO_2.90 to BiFeO₃ from the substrate to the film surface) using the Sol-Gel method on Pt(111)/Ti/SiO₂/Si substrates. After directional distribution of defects within the film being carefully modulated, the BiFeO₃ films are self-polarization when induced by build-in electric field. Piezoresponse force microscopy show that the up-graded and down-graded self-polarization behavior can be modulated by gradient direction of Ca in BiFeO₃ thin films. Moreover, diode-like current-voltage signature verifies the composition gradient-induced self-polarization. The X-ray photoelectron spectroscopy results indicate that the polarization orientation mechanism may arise from the internal electric field attributed to the gradient distribution of oxygen vacancy. This work provides a new strategy to achieve self-polarization in ferroelectric thin films, as well potential novel application in improving the performance of photovoltaic or photosensitive devices as assisted by internal field via self-aligned ferroelectric polarization.